Steel alloys have been used for many years to make engine components such as valves and valve lifters. Although such steel may be suitable in many applications, it has been common practice to provide a high wear resistant surface of specialized steel alloys particularly to the seat face of engine valves and the cam surface of engine valve lifters where such are used in rugged applications such as characteristically associated with gasoline and diesel engines.
The wear resistant surface alloy is characteristically deposited in a molten state upon the steel substrate by a welding process such as a oxy-acetylene or one of the arc welding or fusion welding processes well known to those skilled in the art. Welding processes used for the application of overlay surfaces are characteristically referred to as weld surfacing processes and differ from welding processes only by the deposition of a surfacing material rather than fusing independent members together.
One of the problems associated with welding such surfacing alloys onto steel alloy substrates is that often the steel substrate contains nitrogen which is liberated during the welding process and results in undesirable porosity in the surfacing alloy deposit reducing the surfacing deposit quality and potentially reducing corrosion and erosion resistance.
Two examples of iron based nitrogen containing steel alloys found particularly advantageous for use in making engine valves are described in U.S. Pat. No. 4,547,229, assigned to the assignee of the present invention and the disclosure of which is incorporated herein by reference. The first alloy having a composition similar to S.A.E. EV12 type, contains about 0.36% by weight nitrogen and the second alloy, comprising an iron based austenitic steel, contains about 0.28% by weight nitrogen.
Examples of iron-based alloys useful as casting or pad welding materials are respectively disclosed in Japanese Pat. Nos. 0153872 and 0229470, the disclosures of which are included herein by reference. The alloys however include tungsten in substantial amounts of from 0.1 to 10% by weight as an essential constituent. Tungsten has the ability to form stable carbides in carbon-containing metal alloys and to increase their solid solution strength. Tungsten however is expensive and has approximately twice the atomic weight of molybdenum which is known also for its ability to form stable carbides and to improve solid solution strength of carbon containing metal alloys. It has been discovered that the low porosity surfacing iron-based alloys of the invention herein described can be made without tungsten and therefore eliminate the expense and weight disadvantages associated therewith and, as such, are defined as being "substantially devoid of tungsten" meaning that if tungsten is present in the iron-based alloys of the invention, it is present only as an unavoidable trace contaminate in amounts characteristically less than about 0.01% by weight to the total weight of the alloy.
Another example of iron-based alloys having increased resistance to wear and to corrosion and erosion is disclosed in Soviet Union Pat. No. 0306183, the disclosure of which is included herein by reference. Here however the chromium content is limited to between 18 and 25% whereas the alloys of the invention employ from 25 to 30% and preferably from 26 to 28% by weight chromium in providing the low porosity surfacing alloys of the invention.
Metallic surfacing alloys of the type of interest to the present invention may be iron, chromium, and/or cobalt based but are more commonly nickel based and contain carbon, silicon and chromium in addition to a variety of additional constituents including nickel when the base constituent is other than nickel.
It has been discovered that porosity of the surfacing alloy can be substantially reduced when prescribed amounts of hafnium or niobium and mixtures thereof are added to nickel based surfacing alloys and where prescribed amounts of silicon are added to iron based facing alloys according to the amount of carbon contained in the alloy.
Hafnium (Hf) has an atomic number of 72 and is a quadravalent metal resembling zirconium having both basic and acidic properties. According to the "Encyclopedia of Material and Science Engineering", Michael B. Bern, Permagon Press, 1986, Hafnium is an additive in super alloys due to its ability to absorb neutrons.
Niobium (Nb), formerly known as columbium, has an atomic number of 41 and is found in nature principally as the oxide and has found use as an alloying component in heat resistant alloys, notably stainless steel. Niobium is used as a carbide stabilizer and grain refiner in alloying practice.
Silicon (Si) is well known in steel alloying practice and is reportedly one of the most important dioxidizers in steel making and will react with carbon at above about 1650.degree. C. to form silicon carbide, the well known ceramic. Also well known is silicon nitride which is formed directly as a reaction product between silicon and nitrogen at a temperature above about 1200.degree. C. Although having the known ability to react with nitrogen the ability of silicon to substantially reduce porosity in iron based surfacing alloys has been surprisingly discovered to rely upon the relationship between the amount of carbon and the amount of silicon present in the surfacing alloy and not merely upon the presence of silicon alone without such relationship.